Communications systems employ various means of allowing multiple users to send data streams within an allocated portion of the communications frequency spectrum in a shared manner. These means include time-division multiple access (TDMA); frequency-division multiple access (FDMA); orthogonal frequency division multiplexing (OFDM), with carrier sense multiple access (CSMA); and code division multiple access (CDMA). Performance of these systems can be limited by the canonical algorithmic approaches for frequency spectral analysis and temporal correlation analysis/matched filtering.
Major advances in communications, especially related to cellular telephone systems, have been achieved as a result of spread-spectrum, Code Division Multiple Access (CDMA).
FIG. 1 shows a known generalized CDMA system for explanatory purposes. A binary information sequence of “1s” and “0s” (data bits) is converted to phase-shift forms, e.g., a “1” corresponds to no phase shift, and a “0” corresponds to a 180-degree phase shift. This is referred to as binary phase shift keying (BPSK), although other methods that use more phase shifts and multiple amplitudes, such as QPSK or QAM-64, could be used. These phase shifts can be applied with a particular keying rate to an intermediate frequency subcarrier signal. Next, orthogonal or quasi-orthogonal binary codes (e.g., Walsh Codes and/or Pseudo-Noise PN Codes) are applied in a similar fashion to channelize signals (especially via the Walsh code) and to spread the signal spectrally across a much wider bandwidth than the signal itself requires (e.g., 1.25 MHz for 3GPP2 specifications, and 5 MHz for 3GPP specifications). The signal is modulated onto a radio-frequency carrier signal, amplified, and fed to a transmitting antenna system. Each code is assigned to a different communications channel. With proper orthogonality of the codes, each channel can occupy the same frequency space 100% of the time and still be decoded and despread by receivers. At the receiving end, the RF carrier is demodulated and code correlated with a correlator or a matched filter to despread the signal, leaving an information sequence of 1's and −1's that are re-interpreted as binary ones and zeros. While shown with two steps of frequency conversion, often one step (direct conversion) is used from baseband to RF and vice versa.